When a valuable document such as a bill, valuable security or coupon is inserted into a passageway of a document validator, a conveyor device automatically transports the document along the passageway after the document is aligned (centered) with the passageway so as to bring a longitudinal central line of the document in register with a longitudinal central line of the passageway. A validation sensor is provided at a predetermined location in the passageway to detect physical features such as optical or magnetic features of prescribed areas in the transported document. To align the inserted documents of different width with the passageway leads to exact detection of physical features in correct areas of the document by validation sensor.
For example, a document validator disclosed in Patent Document 1 mentioned below has a centering device that comprises guide rollers in contact to a conveyor belt for transporting an inserted bill along a passageway and movable between the contact position and spaced position away from conveyor belt, and a pair of pinch jaws of channel-shaped section moved toward each other to grip opposite sides of bill in passageway so that pinch jaws make a longitudinal central axis of bill come coaxial with longitudinal central axis of passageway. When centered, bill produces extremely increased resistance against the buckling by pinch jaws due to stiffness of bill, and therefore, a rotor of a centering motor arrives at a power-swing damping or slippage to forcibly hinder further rotation of centering motor when increased resistance over a predetermined level is applied to centering motor. At the moment, operation of centering motor is ceased to stop movement of pinch jaws. Then, centering motor is driven in the adverse direction to return pinch jaws away from bill to the original outermost position, and guide rollers are returned from the separated position to the contact position to bring guide rollers into contact to bill which are therefore further inwardly moved along passageway to detect authenticity of bill by a validation sensor.    [Patent Document 1] Japanese Patent Disclosure No. 2005-115811